Problem: On the first 4 tests of her physics class, Umaima got an average score of 78. What does she need on the next test to have an overall average of 81?
Answer: Let her score on the next test be $x$ The sum of all of her scores is then $4 \cdot 78 + x$ The same sum must also be equal to $5 \cdot 81$ Solve: $x = 5 \cdot 81 - 4 \cdot 78 = 93$.